Heart failure is a leading cause of mortality worldwide, and effective therapies to repair damaged cardiac tissue are badly needed given that myocardial regeneration is clearly inadequate in the setting of extensive injury. A critical intervention in the loss of function could be to repopulate cardiomyocytes lost as a result of the ischemic injury. The development of therapeutic strategies to stimulate endogenous cardiomyocyte proliferation could treat and/or prevent heart failure. Recent data suggest that the EGF family member neuregulin-12 (NRG) can improve survival and cardiac function in models of in vivo ischemic cardiomyopathy potentially by cardiomyocyte division. However, because the mechanism by which NRG improves cardiomyopathy outcomes is not fully understood, the full therapeutic potential of NRG remains unrealized. NRG binds its receptor ErbB4 with high affinity in the heart and induces predominantly ErbB2/ErbB4 heterodimerization, as well as ErbB4 homodimerization, leading to distinct signaling outcomes. My preliminary data suggest that biasing ErbB receptor signaling leads to different signaling in cardiomyocytes. Therefore the objective of this proposal is to utilize molecular design, protein biochemistry and in vivo molecular biology to engineer NRG ligands that bias receptor dimerization and determine their signaling outcome in the myocardium. This objective will be accomplished through two primary aims. In Aim 1, I will determine whether biasing ErbB receptor dimerization with engineered NRG ligands induces adult cardiomyocyte proliferation. In vitro findings will be confirmed with in vivo mouse models that incorporate inducible cardiac-specific deletion of ErbB receptors to establish molecular mechanisms at the receptor level, and genetic fate-mapping approaches will be used to determine if biasing ErbB signaling affects established cardiomyocytes or progenitor cells. In Aim 2, modified NRG ligands will be tested for their ability to repair the myocardium following infarction injury in vivo; this experiment will directly address a crucial clinically-relevant hypothesis. Collectively, these studies will contribute to understanding NRG-induced ErbB receptor dimerization and its effects on cardiomyopathy outcomes. In addition to fundamental mechanistic insight into ErbB signaling in the heart, these experiments have the potential to reveal a novel therapeutic strategy for cardiac regeneration. This project will also provide the crucial training in molecular biology and in vivo experimentation that I need to complement my undergraduate and graduate work in engineering, so that I can pursue a career in investigation that will translate bioengineering approaches into in vivo benefits. These advances would enable the optimization of NRG ligands to potentiate their effectiveness for repair of myocardial infarction injuries.